1. Field of the Invention
This invention relates to methods of making thermoplastic tubular articles which are dimensionally stable up to a specific elevated temperature from crystallisable polymers, and to the apparatus for making such articles, which articles are particularly tubular bodies of saturated linear polyester materials such as polyethylene terephthalate, intended for processable food and beverage containers.
2. Description of the Prior Art
It is known that biaxial drawing of a thermoplastic saturated linear polyester material, such as polyethylene terephthalate, can improve its mechanical properties while inducing a degree of biaxial orientation and crystallisation, without impairing the clarity of the material. The material will, however, shrink if heated above the temperature at which it was drawn. It is known that the tendency of biaxially drawn polyester film material to shrink can be decreased by annealing the material under restraint, utilising temperatures in the range 150.degree. C. to 230.degree. C., the process being known as heat setting. Biaxial drawing is also effected, for example, in the stretch-blow moulding of polyethylene terephthalate bottles. Another method of forming biaxially oriented tubular articles which may be used as bodies for processable food containers is disclosed in our copending U.K. Patent Application No. 8037137, published under No. 2089276A. The forming process described therein involves longitudinal stretching and radial expansion under internal fluid pressure within a mould and is normally carried out at a temperature in the range from 75.degree. C. to 120.degree. C. The tubular bodies thereby produced are dimensionally stable up to the temperatures encountered in many container filling operations, but a heat-setting treatment is necessary if the containers are to be filled with hot product (e.g. at 80.degree. C. to 100.degree. C.) or are to be subjected to pasteurisation (at about 60.degree. C. to 100.degree. C.) or sterilisation (at about 120.degree. C.). Without such a heat-setting treatment, the thermoplastic linear polyester material (e.g. polyethylene terephthalate) would shrink and distort to an unacceptable degree during the hot filling or processing, as a result of relaxation of strained portions of the material back into their preferred state in which the molecular chains are coiled rather than extended. In the heat-setting treatment described in Specification No. 2089276A, the tubular articles were held for a time at a temperature at least equal to, and preferably somewhat greater than, the sterilisation or pasteurisation temperature whilst restraining them from shrinking beyond their desired circumferential and longitudinal dimensions. The restraint was provided by internal fluid pressure, so as to set the strained, oriented amorphous portions of the polyester material by at least partial crystallisation and relaxation. Internal fluid pressure cannot be conveniently used when the tubes have been cut so as to be open at both ends, and it is not convenient to combine the heat-setting with the forming process of our Specification No. 2089276A because it would involve additional complication and extended process times.
European patent application published under No. 0081451 discloses a heat-setting process and apparatus for making closed-ended containers from PET. The process comprises deep-drawing a pre-heated sheet of PET into a heated female mould by means of a male plug, which is at a temperature below the glass transition temperature (Tg) of the PET. The mould is kept at a temperature above Tg, typically 140.degree. C. Air pressure introduced around the plug is used to blow the plug-formed article into contact with the mould for heating it above Tg. When it has been in contact with the mould for long enough to be heat-set, it is allowed to shrink back into contact with the plug which restrains it from further shrinkage and cools it to a temperature below the heat-set temperature. Contact between the heat-set article and the plug can be released by forcing air through an axial passage in the plug. In this process the heat-setting is essentially carried out while the article is held against the female mould by internal gas pressure, rather than when it is restrained from axial shrinkage by the plug. The heat-set operation cannot be carried out independently of the biaxial drawing, and is clearly not applicable for heat-setting open-ended tubes.